Meeting Abstract
Previous research has shown that triceps surae mechanical advantage is typically highest in juvenile mammals and scales with negative allometry during growth. Because lower triceps surae mechanical advantage increases elastic energy storage in the Achilles tendon, improving locomotor performance, this suggests selection for force production in juveniles versus selection for locomotor economy in adults. We test this hypothesis using eastern cottontail rabbits (Sylvilagus floridanus) as a model system. We found that maximal ankle joint moments scale with isometry during growth (M1.31; R2=0.86), whereas proximal calcaneal length (i.e., triceps surae moment arm length) scales with negative allometry (M0.23; R2=0.87), suggesting that the maximal tensile force imparted to the Achilles tendon should scale with positive allometry (M1.08). All else being equal, increasing force should increase tendon strain, improving the potential for elastic energy storage among adults. However, because the cross-sectional area (CSA) of the Achilles tendon scales with positive allometry (M0.79; R2=0.87), maximal tendon stress scales with slight negative allometry (M0.29). These data suggest that, contrary to our hypothesis, tendon strain may actually decrease during ontogeny, particularly if adult tendons have greater elastic modulus, as previously shown in laboratory rabbits. Negative allometry of muscle lever lengths, combined with positive allometry of tendon CSA, may permit juvenile rabbits to optimize both force production and energy storage at the ankle, increasing locomotor performance despite small body size. Adults, by contrast, appear to optimize tendon safety factor in lieu of locomotor economy. Supported by NSF IOS-1146916.
